Not really much originality here, but I mean what can you really do to make an original "human killing machine" that also has a supposingly cool form?

In this case he just has more weaponry.
He stops us at the end of The Core. He is an optional boss since the other choice is to fight Caulen in his Mansion to proceed.
The Core however is not the same as in Undertale. It works and generates power from heat, but the areas inside that we normally explore collapses sometimes and it's generally a place people are scared to go in, it's abandoned, lights are off and most things are turned off, except puzzles which Alphys reactivates before we get there.
On pacifist he will probably genuinly wish us to not leave or give us a final challenge before facing last guards and The King himself. (I have no plans for changes of the battle because of the route, except dialogue after or inside of the fight when he stops his acting in front of audience and says the truth. Perhaps he could do it because he knows that Frisk befriended a lot of people from his audience and MTT isn't "bad" inside even if he can seem cruel or even rampaging.)
On neutral he'll fight us sort of forced by Alphys, also to impress the audience at some point and give them hope for freedom, to stop us and just take our soul (although the number of souls is unknown to anyone except certain guards like Undyne).

On genocide Frisk will fight Mettaton NEO of course. That route will change a lot, so if I provide any information about it, that will be when I have the sprite and the idea for it.